In certain situations it is useful to make precise determinations of the moisture content of materials. The food processing industry is one which has such needs; and, for purposes of illustration, the present invention will be described in that environment.
In the food processing industry, governmental regulations often fix a stringent limit on the minimum or maximum amount of moisture that is permitted to be present in a given product. In all cases, it is desirable to be able conveniently to guide the composition of food materials as to moisture content in order that tight control of processing can be maintained to assure efficient operation and efficient use of resources.
Heretofore the moisture content of liquid, semi-solid, or powdered food material has been determined by a variety of methods, including, for example, a gravimetric method, utilizing either a vacuum or a microwave oven [i.e., Association of Official Analytical Chemists (AOAC) Method 1990]. Both variations are inconvenient to use when regular measurement of the moisture content of, e.g., hundreds of samples is required. The vacuum oven variation further requires undesirably long measurement times for on-line food processing. Some other methods include measuring reflectance or absorption or some other parameter as an indirect indication of moisture content. However, all prior techniques have been found to lack at least one of several requirements for conveniently and meaningfully measuring moisture content of food materials. Such requirements include, illustratively, high temperature tolerance [e.g., up to about 120.degree. C.], short time [preferably less than about five minutes] to make a moisture content measurement of a sample, ease of calibration, and substantially no contamination between sensor and food system.
It has long been known that moisture content has a strong influence on thermal conductivity of materials. This is shown, for example, in an F. C. Hooper et al. paper "Transient Heat Flow Apparatus for the Determination of Thermal Conductivities" which appeared in Heating Piping & Air Conditioning, Aug. 1950, pages 129-134. In Hooper et al., a probe containing a line heat source and thermocouple junctions was utilized to make temperature measurements in a variety of materials, including soil. Heating time used was short enough that moisture content of a sample did not change. Corrections were made for the finite diameter of the probe, and temperature and heating current were measured to three significant figures. Thermal conductivity was determined from the slope of the temperature rise in a sample versus the natural logarithm of heating time in the time interval between four and ten minutes after the start of heater current.
A number of workers have tried to correlate temperature response properties as a function of moisture content, but none has been able to satisfy all of the above mentioned requirements for use in food processing systems. Some examples of this type of work are noted below.
J. E. Lozano et al. reported in "Thermal Conductivity of Apples As A Function of Moisture Content," Journal of Food Science, Vol. 44, No. 1 (1979), pages 198-199, the measurement of temperature rise in respective apple samples having different known moisture contents. Each sample was heated for a known heating time and at a known level, and the thermal conductivity was calculated as a function of the measured temperature difference.
V. E. Sweat used a thermal probe to measure thermal conductivities of various fruits and vegetables in an effort to determine whether or not thermal conductivity could be estimated using water content and temperature of a sample. "Experimental Values of Thermal Conductivity of Selected Fruits and Vegetables," Journal of Food Science, Vol. 39 (1974), pages 1080-1083. The author concluded that there was a sufficient correlation between water content and thermal conductivity of certain fruits and vegetables that a linear regression equation could be used to predict thermal conductivity as a function of water content to within .+-.15%.
A U.S. Pat. No. 4,845,978 to D. R. Whitford discloses, for purposes of controlling an irrigation system, that the time rate of temperature rise in soil could be considered to be an approximate indication of moisture content in the soil. Whitford employed a thermal probe including a point heat source and a spatially separated temperature sensor. However, the technique requires heating for a much longer time interval than can be tolerated for most foods, especially those which must be processed at elevated temperatures ranging over 100.degree. Celsius [C.]. Furthermore, a straight-line approximation of a temperature versus heating-time curve, such as taught in Whitford, has been found to lack sufficient precision for use in processing many food materials.
Several prior workers have exploited a relationship between moisture content and some parameter, other than temperature, by applying linear regression analysis to obtain moisture-related information about a sample. For example, a U.S. Pat. No. 4,310,758 to J. R. Peterson teaches the direct measurement of bidirectional reflectance factors as to spectral reflectance for soil and utilization of those data in predictive equations, determined from regression analysis, to solve for moisture tension (vis-a-vis moisture content). Similarly, a U.S. Pat. No. 4,568,875 to J. S. Piso et al. teaches the direct measurement of yarn denier and utilization of those data in certain equations determined with the aid of regression analysis to solve for moisture content of yarn samples having unknown moisture content. In yet another case, U.S. Pat. No. 4,651,285 to M. J. Collins et al., fat content of certain food materials is determined by measuring density and measuring solids content and utilizing that data in equations, determined by regression analysis, to solve for fat content of samples having unknown fat content. It is stated in Collins et al. that a measurement of moisture content can be employed instead of solids content, and presumably that statement is based upon a prior statement in the patent that it is known to determine moisture content by a gravimetric method including heating by microwave energy. None of the foregoing methods meets all of the requirements for conveniently and meaningfully determining moisture content of food materials.
Another problem associated with moisture content measurements based upon thermal characteristics of materials being measured is that heretofore sample material has been installed in a sample holder, and thereafter a thermal probe has been assembled into the holder. Since an operator is unable to monitor visually the probe-sample interface for the full extent of the probe length during probe installation, there can be undetected nonuniformities in the interface between the sample material and the probe. Such nonuniformities distort heat transfer across that interface and, therefore, the temperature measurements being made.
Yet another difficulty associated with moisture content measurement based upon thermal characteristics of materials being measured arises from the fact that the density of a powdered material sample is known to have a strong effect upon thermal response measurements. Known techniques such as, e.g., tapping a sample holder to cause the material to settle or gas injection to measure the extent of voids in the sample holder, are either lacking in precision [tapping] or unduly time consuming [gas injection].